1. Field of the Invention
The present invention relates to composite drill pipes and the method for forming same, and more particularly to composite pipe segments bonded to metal end fittings for joining thereof into a pipe string, wherein each fitting may include bridging connections for transmission of signal and/or power carried on leads imbedded in the composite.
2. Description of the Prior Art
As the known formations containing oil and/or gas are depleted drilling for new deposits is invariably at a greater depth and/or greater lateral reach, with the depth and reach limits determined mainly by the cumulated weight and unit strength of the pipe string. Traditionally, the use of steel pipe has fixed these weight-to-strength ratios, setting a limit that has been particularly bothersome in ultra-deep, deep directional and/or extended reach drilling caused either by the weight of the pipe or by the weight induced friction of the rotating pipe string as it rests on the walls of the well bore, or rubs against the casing wall. The increasingly large expenses of remote drilling or drilling from a platform, moreover, impose some rigor in directional response which is best effected by drill pipe capable of short turning radii, again a facility that is difficult to obtain in steel pipe. The drilling entrepreneur, therefore, has been limited for some time by the stiffness; the unit weight; and the fatigue, shear and tensile strength limits of steel pipe. As a result various material alternatives have been proposed both to reduce the linear weight and flexure of the string and to improve its fatigue, shear and tensile limits. This search for light weight and high strength material substitutes has led to composite pipe structures, particularly since composites also offer the added benefits of being more resistive to corrosion.
Composites, however, are less effective at forming mechanical joints while virtually all drilling operations require limited length pipe segments, determined by the size of the drilling rig and/or the handling power of any lifting equipment, and the step of joining such pipe segments into a long string is a fundamental aspect of all drilling. For this reason the more recent development focus has been directed to the interface between the composite wall of the pipe and metal end fittings on each end of the segment.
Composite materials have a further advantage that heretofore has not been extensively recognized, namely the convenient imbedding of signal and/or power conductors into the laminates forming the pipe wall. This function is particularly useful with short radius directional drilling as it allows for an uninterrupted, continuous down hole signal feedback and also control augmentation while drilling, thus maximizing the effectiveness of the invariably very high drilling costs at the remaining remote or deeply submerged formations. This synergistic aspect of composite pipe has not been fully recognized nor exploited in the art, simply because the technical challenge of forming an effective connection between the composite tube wall and the end fitting has overwhelmed all other considerations. The process of imbedding conductors or connecting them across a joint has therefore been relegated to inattention.
At the core is the inherent difficulty in forming a high integrity interface between the composite pipe wall and the joining surfaces of the end fitting. In the past fitting assemblies with variously opposing surface geometries have been proposed to effect a secure capture of the composite end within the fitting. Some examples of such end fittings include those taught in U.S. Pat. No. 5,233,737 to Policelli; U.S. Pat. No. 4,810,010 to Jones; U.S. Pat. No. 6,315,002 to Antal et al.; and others. While suitable for the purposes intended each of the foregoing assemblies include threaded or otherwise releasably engaged parts clamping the composite between each other with inherently uneven load concentrations resulting in highly uneven shear stresses. This uneven load distribution between adjacent parts, of course, results in correspondingly uneven local strain deformations when exposed to the various high loadings in the course of use. There is therefore an inherent incidence of local bond separation between the composite itself and the adjoining fitting surface, with some consequence for failure.
Alternatively, end fitting assemblies have been proposed in which radial pins or other radial fasteners are added to the assembly, as exemplified by the teachings of U.S. Pat. No. 5,332,049 to Tew; U.S. Pat. No. 5,288,109 to Auberon et al.; U.S. Pat. No. 5,443,099 to Chaussepied et al.; and others. Once again, while some improvement in structural integrity is realized from these radial interconnections the essentially separated nature of the assembly components is also susceptible to uneven load transfer with the consequent local separations an inherent possibility.
An end fitting structure conformed for bridging interconnection of imbedded leads and also adapted for assembly that results in a single, integral structure clamped and bonded to the composite is extensively desired and it is the implementation of one such end fitting that is disclosed herein.